1. Field of the Invention
The present invention relates to a wiring material of semiconductor devices. Specifically the present invention relates to a semiconductor device having a circuit comprising a thin film transistor (hereinafter referred to as TFT), and a manufacturing method thereof. For example, the present invention relates to an electro-optical device, typically a liquid crystal display device, and an electronic device with an electro-optical device installed as a component.
Note that through this specification, a semiconductor device indicates general devices that can function by using semiconductor characteristics, and that electro-optical devices, semiconductor circuits, and electronic devices are all categorized as semiconductor devices.
2. Description of the Related Art
Techniques for using semiconductor thin films (with a thickness on the order of several nm to several hundreds of nm) formed over a substrate having an insulating surface to structure a thin film transistor (TFT) have been in the spotlight in recent years. Thin film transistors are widely applied to electronic devices such as ICs and electro-optical devices, and the rapid development thereof as switching elements for image display devices is desired.
Conventionally, aluminum films formed by sputtering and having low resistivity have been often used as the wiring material for the above stated TFTs. However, when a TFT is manufactured by using aluminum as a wiring material, operation error or deterioration of TFT characteristics were caused by formation of projections such as hillocks or whiskers or by diffusion of aluminum atoms into the channel forming region, in the heat treatment.
As stated above, aluminum is not a preferable wiring material in the TFT manufacturing process because of its low heat resistance.
Therefore, materials containing, for example, tantalum (Ta) or titanium (Ti) as a main constituent are being tested for use as a wiring material other than aluminum. Tantalum and titanium have a high heat resistance in comparison to aluminum, but on the other hand a problem of high electrical resistivity develops. Further, if heat treatment is performed at a temperature on the order of 500° C., it becomes a problem that the electrical resistance increases by several times compared to before heat treatment.
Furthermore, when a film formed on a substrate possesses a large stress, substrate warping and film peeling generate, so it is preferable to perform film stress control and to form a film which possesses as low a stress as possible for a film formed by sputtering. The use of a gas mixture of argon (Ar), krypton (Kr), xenon (Xe) as a sputtering gas has been proposed as one means of performing control of film stress. However, krypton (Kr) and xenon (Xe) are high priced, and therefore use of the gas mixture is unsuitable for cases of mass production.